1. Field of the Invention
The present invention relates to a asynchronous mobile communication system, and more particularly to a cell searching apparatus and method in an asynchronous mobile communication system.
2. Background of the Related Art
In an asynchronous mobile communication system, a mobile terminal may use a primary synchronization channel (P-SCH) and a secondary synchronization channel (S-SCH) among downlink physical channels (DPCH) in order to search for a base station. The base station can transmit a primary synchronization code (PSC) through the primary synchronization channel and a secondary synchronization code (SSC) through the secondary synchronization channel.
Each frame generally used in the asynchronous mobile communication system has a period of 10 ms and includes 15 slots (#0-#14). The base station transmits the PSC and SSC for every slot. If each slot has 2560 chips, a sequence length of the PSC of the P-SCH is 256 chips and can be positioned at the first 256 chip period of each slot. The SSC of the S-SCH has the same value for every frame.
Thus, generally, slot synchronization can be accomplished using the PSC of the PSCH and frame synchronization can be performed using the SSC of the S-SCH based on the slot synchronization. At the same time, a value of the scrambling code group (SCG) can be determined.
Each base station is assigned one of 512 scrambling codes (SC) for use and the mobile station should first determine a scrambling code group (SCG). The 512 scrambling codes are classified into 64 scrambling code groups, each group corresponding to 8 scrambling codes. Searching the base station (e.g., the cell), signifies finally detecting which scrambling code a cell to which a mobile terminal belongs uses.
FIG. 1 shows a general asynchronous mobile communication system. This system includes a mobile terminal 11 for performing base station searching through a radio access, base stations 12A-12N for connecting the mobile terminal to a mobile communication network 14 through a radio interface, and a radio network controller (RNC) 13 for controlling the base stations 12A-12N. In the general asynchronous mobile communication system constructed as described above, the SCG is assigned to each cell randomly without any regulations.
FIG. 2 shows a general cell searching apparatus of a mobile terminal. This apparatus includes: a filter 21 for removing a noise component contained in a radio frequency (RF) signal; a slot synchronizer 22 for detecting a PSC with the most powerful reception strength among a plurality of PSCs outputted from the filter 21 and for performing a slot synchronization (SS); a frame synchronizer 23 for detecting a SSC with the most powerful reception among a plurality of SSCs outputted from the filter 21 based on the slot synchronization, performing a frame synchronization, and determining an SCG; and a scrambling code detector 24 for detecting an SC by using the determined SCG.
Operation of the general cell searching apparatus of a mobile terminal will now be described. When the mobile terminal 11 is powered on, the mobile terminal starts searching a cell (e.g., a base station), which can provide service to the mobile terminal. The terminal then receives PSCs and SSCs transmitted from the base stations 12A-12N.
The slot synchronizer 22 of the mobile terminal detects a signal (PSC) with the most powerful reception strength during the slot period, and then if the signal is detected again after the slot period, the slot synchronizer 22 performs a slot synchronization (SS) based on the reception timing of the signal. The reason for using the PSC signal with the most powerful reception strength is because the reception strength of the PSC signal transmitted from the base station where the mobile terminal belongs is the most powerful.
When slot synchronization is performed, the frame synchronizer 23 searches the SSCs outputted from filter 21 based on the slot synchronization. If an SSC with the most powerful reception strength is detected, the frame synchronizer 23 performs a frame synchronization based on the detected SSC and determines an SCG. The scrambling code detector 24 detects an SC of the base station to which the mobile terminal belongs using the SCG.
In the general asynchronous mobile communication system, the SCG is randomly assigned to each base station. Thus, even if mobile terminal 11 is aware of the SCG of base stations adjacent to a specific base station to which the mobile terminal 11 belongs, it cannot know with any certainty the SCG of the specific base station. Thus, in searching the SSCs, since frame synchronization has not yet been performed, every case of shifting as many times as the number of slots (i.e., 15 slots) pet frame can be candidates of the frame synchronization and the SCG. Therefore, the frame synchronizer 23 of the mobile terminal 11 must repeatedly perform the SSC detecting process 960 times (15*64) in the worst case, in order to obtain frame synchronization and SCG.
The general cell searching apparatus of the mobile terminal, thus, has problems in that the cell searching rate is degraded and power consumption is high due to the load for cell searching.